The present invention relates to ink jet printing, and more particularly to printing with ink projected from an oscillating nozzle.
Prior art devices for recording with liquid ink may be categorized into those involving continuous or sporadic contact between a stylus and a recording medium, and those involving projection of ink onto a recording surface. The latter devices, known as "ink jets", may further be classified as to the matter in which the flow of ink is regulated, and as to the method by which the ink is targeted onto the recording medium. Ink flow is generally regulated through electrical means. By contrast, a variety of techniques have been utilized in directing the ink stream.
One ink drop targeting method, disclosed for example by Sweet in U.S. Pat. Nos. 3,373,437 and 3,596,275, and by Lewis in U.S. Pat. No. 3,298,030, involves charging the ink jet electrically and then deflecting it to the desired position on a record member. In this method, an ink stream is emitted from a nozzle which directs the stream generally toward a record member. Individual ink drops are quantitatively charged in an electrode placed near the point at which the jet breaks up into individual droplets. The size and separation of the droplets, and point of breakup, must be regulated carefully (typically by excitational means appended to the nozzle), as the mass and volume of the droplets will affect their eventual placement on the record member. The droplets pass through high voltage DC deflection electrodes, which create an electrical field surrounding the ink jet. The droplets are deflected within a plane according to the magnitude and polarity of charge received in the charging electrode. An extreme degree of deflection within this plane will cause droplets to be directed to an intercepting member. The above process of forming an image raises several difficulties. Each drop must be given a charge level which reflects not only its intended location on the record member, but also the charge levels given several previous drops (due to electrostatic interactions between the droplets). This requires quite complicated controlling electronics. Furthermore, as a one level targeting system (relying solely on electrostatic means to form the image), this system is relatively sensitive to noise due to electrical interference.
Another jet directing process, described by Hertz and Simonsson in U.S. Pat. No. 3,416,153, share with the above the features of an ink emitting nozzle and a charging electrode. In this method, however, the jet is charged to a degree that causes it to diffuse and form a spray. Diffused droplets (i.e. all but those in the undiffused jet path) may be prevented from reaching the record member by DC deflection electrodes similar to those of Sweet and Lewis, or by an apertured interception member.
A third method of image formation with an ink jet involves the projection of ink from an oscillating nozzle, with a record member which moves continuously in a direction essentially transverse both to the axis of the nozzle and the axis of oscillation. The result is that the ink jet traces a sinusoidal scanning pattern, and the image may be controlled by such factors as the spread of ink drops as they impinge on the recording surface, whether the ink jet is on or off, and the frequency and amplitude of oscillation.
The parameters of the frequency and amplitude of oscillation are subject to physical limits defined by the device employed to induce nozzle oscillation. One such device is comprised of a galvanometer attached near the tip of a glass capillary tube, with the tip bent at a right angle. The periodic torsion induced by an AC current through the galvanometer windings causes the nozzle tip to oscillate. Such a device is disclosed, for example, by Elmquist in U.S. Pat. No. 2,566,443. A range of frequencies may be obtained through this device, typically with a one to two KHZ upper limit, and no lower limit. This broad band device involves serious control problems, however, in that there is a phase lag at high frequencies between the actual location of the galvanometer and that perceived by a control mechanism, and this phase lag varies from frequency to frequency, making this device quite difficult to calibrate.
An alternative approach which avoids this drawback achieves the desired oscillation by means of a mechanically resonant structure, such as a vibrating metal reed. The reed carries a capillary tube from which the ink is projected, with the oscillations of the device confined to frequencies at or near the resonant frequency of the reed. The oscillation of the reed is induced by electromagnetic means. An example of an ink jet system of this type is disclosed by Hertz in U.S. Pat. No. 3,737,914. To produce a desirably high amplitude of nozzle oscillation, it is necessary in practical terms to drive the resonant structure at or near resonance. At significant deviations from the resonant frequency (i.e. outside the bandwidth), a substantial increase of current in the electromagnetic excitational means is required to produce the same amplitude of oscillation. This would entail overly high demands on the excitational means, and also involve a danger of saturation of the magnetic material of the metal reed. Driving such a structure at resonance, however, involves problems such as ink buildup on the reed, which causes a drifting of the frequency of resonance.
The drifting of the frequency of oscillation also raise problems in keeping the drop charging means in a correct phase relationship with the nozzle oscillations. One solution, which appears in Hertz U.S. Pat. No. 3,737,914, is to use an optical sensor to detect the position of the resonant structure, and use a signal generated thereby to regulate the charging process. Similarly, other transducers may be used for this purpose. These devices, however, often encounter operational difficulties when used in the vicinity of the ink stream.
Vibrating reed ink jet systems, and similar systems, also encounter difficulties at printing speeds higher than 100 characters per second. At high printing speeds, print quality tends to be inconsistent. Furthermore, printing of acceptable quality with a vibrating reed system has required the recording medium to pass within 1/8 inch of the electrode assembly which is used to control the flight of the ink drops. As a result, the printing of curved or recessed surfaces is difficult and often impossible.
In the vibrating reed system, the problems of inconsistent print quality and the low effective range of the ink jet stream may both be traced to the haphazard separation of the ink stream into individual drops. Various devices have been incorporated in other ink jet systems, such as those of Sweet and Lewis, with a view to regulating ink drop formation. A method generally employed in this regard involves inducing a pulsation of the ink stream at a controlled frequency. This has sometimes been accomplished by means of a transducer which is incorporated into the capillary structure, and placed in intimate contact with the ink stream, perhaps separated by a membrane. These devices, however, possess the shortcoming that the vibrations by which the ink stream pulsation is produced have transverse as well as longitudinal components with respect to the axis of the capillary tube. They therefore interfere, to some extent, with the transverse oscillations of the nozzle by which the scanning pattern is created in the third type of ink jet system. This effect is especially pronounced at higher printing speeds (higher frequencies of transverse nozzle oscillation).
The ability to print at higher speeds is correlative to a higher attainable frequency of character generation, which in turn is dependent on the resonant frequency of the reed. Speed limitations mentioned above with respect to prior art ink jet systems employing the scanning nozzle may be identified with a characteristic upper frequency limit of nozzle oscillation of around 1 KHz. For a vibrating reed system, a higher resonant frequency demands a shorter metal reed. Furthermore, to avoid high current demands in an electromagnetic means for inducing reed oscillation, a thin reed is desirable. Prior art vibrating reed systems have not incorporated a reed with these properties, as such a system necessitates a more compact design, and poses more rigorous construction tolerances in avoiding spurious resonances which might interfere with the high frequency reed oscillation.
An ink jet system incorporating an oscillating nozzle requires some means for gating ink drops, as the desired image is created by printing with selected drops. One type of gating system is that used by Sweet and Lewis, a charging electrode in combination with DC electrodes. Unlike Sweet and Lewis, however, these are used in an on/off mode, merely to remove unwanted droplets from the jet and allow the others to continue in a trajectory determined primarily by the nozzle oscillation. While the charging and deflection processes are not as critical as in Sweet and Lewis, they must nevertheless be carefully controlled to produce a high quality image.
Accordingly, it is a principal object of the invention to provide a workable ink jet system of the oscillating nozzle type. Related objects are a simplicity in the controlling electronics, and relative immunity to electrical interference. A subsidiary object of the invention is to achieve an oscillation of the ink jet nozzle by utilizing a magnetically resonant structure, such as a metal reed. A related object is the avoidance of phase calibration difficulties in the means for controlling nozzle oscillation.
Another object of the invention is the achievement of control over the frequency of nozzle vibration to ensure oscillation at or near the lowest resonance frequency of a metal rod. A related object is the avoidance of high current demands in electromagnetic means for inducing reed oscillation. Another related object is the inclusion of means to coordinate the ink jet gating means with the position of the oscillating nozzle. Such coordinating means should be operationally compatible with a liquid ink jet system.
It is a further object of the invention to provide ink jet printing at increased rates of speed. A correlative object is to increase the rate of character generation. A related object is to design a vibrating reed ink jet printing system incorporating a short, thin vibrating metal reed. Another related object is to design a more compact ink jet system which is compatible with a shorter metal reed.
Yet another object of the invention is to achieve ink jet printing with improved quality of print at higher printing speeds. A further object of the invention is to increase the effective range of the ink jet stream. A related object is to permit printing of acceptable quality upon materials at a greater distance from the ink jet nozzle. Another related ojbect is to make possible the printing of curved or recessed materials.
A secondary object which is related to the above objects is to regulate the flow of ink by creating a uniform ink drop size and spacing while controlling the location of ink jet breakup. A further related object is to employ means for this purpose which will not interfere with the transverse oscillation of the ink jet nozzle.
Still another object of the invention is the provision of means to charge selected ink droplets in order that certain droplets will be removed from the ink jet before reaching a record member. The charging and deflection processes should be carefully controlled to ensure an image of acceptable quality.